Update firefox config to point to base-browser branch rather than a particular tag in nightly

With 12.0a5 we'll be switching over to having a separate base-browser branch as well as build tags.

changed milestone to %Sponsor 131 - Phase 3 - Major ESR 102 Migration

added Sponsor 131 FF102-esr labels

assigned to @richard

added Doing label

assigned to @pierov and unassigned @richard

marked this issue as related to #40668 (closed)

mentioned in merge request !588 (merged)

closed with merge request !588 (merged)

removed Doing label